1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method and more particularly to an image forming apparatus and an image forming method capable of obtaining a satisfactory transfer property.
2. Description of the Related Art
Recently, an image forming apparatus having an image carrier and a transfer member opposite to it and a transfer process of passing a transfer material between the two by impressing a transfer voltage to the transfer member, thereby transferring toner on the image carrier onto the transfer material has been proposed. In such an image forming apparatus, as an impressing method of the transfer voltage impressed to the transfer member, a constant-voltage control system and a constant-current control system are known.
In the case of the constant-voltage control system, in an N/N environment (an environment at 23° C., and 55% RH), toner can be transferred appropriately, while in an L/L environment (an environment at 10° C. and 20% RH), the resistances of the transfer material, transfer member, and image carrier increase, and a necessary transfer current cannot be obtained, thus defective transfer may occur. On the other hand, in the case of the constant-current control system, in both N/N and L/L environments, toner can be transferred appropriately, though another problem arises. Namely, for example, if the maximum width of transfer materials is the size of A3, when transferring toner onto a transfer material whose width is narrower than the maximum width of transfer materials such as the size of A4-R, since the width of the transfer material is narrow, the transfer member directly makes contact with the image carrier, and most of the current is carried through the part coated with no transfer material, and no current is carried through the part with the transfer material coated, thus no necessary transfer current is obtained, causing defective transfer.
Accordingly, for example, as shown in Japanese Patent Application Publication No. 2-264278, a control system in combination of the constant-voltage control and constant-current control so as to allow a transfer belt to make contact with a transfer roller when no sheet of paper (transfer material) is loaded, execute constant-current control for the transfer roller, thereby measure a voltage V1 generated on the transfer roller, and when actually transferring onto a sheet of paper, execute constant-voltage control at a voltage of V2 higher than V1 is proposed.
According to the control method proposed in Japanese Patent Application Publication No. 2-264278, in consideration of the transfer bias partial voltage due to the resistance between the sheet of paper and the toner, V2 is decided by multiplying V1 by a predetermined coefficient R, so that in every environment and regardless of change in the size of transfer materials, a stable and satisfactory transfer property can be obtained always.
Further, for example, as indicated in Japanese Patent Application Publication No. 8-190285, a method for forming a toner image on the surface of a photosensitive drum, directly impressing a bias voltage due a constant current to the surface to detect a voltage V1, allowing the surface of a transfer material to make contact with the image forming area and marginal area of the surface of the photosensitive drum, impressing the bias voltage due to the constant current to the part of the rear of the transfer material opposite to the marginal area to detect a voltage V2, calculating a predetermined transfer voltage on the basis of these voltages V1 and V2, and impressing the calculated transfer voltage to the part opposite to the image forming area of the rear of the transfer material is proposed.
According to the control method proposed in Japanese Patent Application Publication No. 8-190285, the transfer voltage is calculated in consideration of not only the resistance of the transfer means such as the transfer roller but also the resistance of the transfer material. By doing this, for transfer materials of various kinds and weights, in various kinds of environments, a stable and satisfactory transfer property can be obtained.
However, according to the control methods proposed in Japanese Patent Application Publication No. 2-264278 and Japanese Patent Application Publication No. 8-190285, the resistances of the transfer roller, transfer belt, transfer material, and toner are measured or estimated correctly, thus an appropriate transfer voltage can be set. However, the transfer voltage varies with the magnitude of the charge amount of toner, so that if the charge amount of toner is shifted from the ordinary value for some reason, a problem arises that an appropriate transfer voltage cannot be impressed.